1. Field of the Invention
This invention relates to a mixer priority architecture and, more particularly, to a mixer priority architecture which is usable with image signal processors to eliminate a soft border transition artifact so commonly encountered with known image processors.
2. Description of Related Art
Known image processors of the type capable of switching image signals are sometimes called "switchers" in the art. For example, U.S. Pat. No. 4,853,784, which issued Aug. 1, 1989 and which is entitled "Video Switcher with Independent Processing of Selected Video Signals," describes a switcher with several video input source signals, which are separately selectable. A selected video input signal is processed by a channel processor. It is common that a key signal, which is associated with a video input signal, will be modified during the channel processing. The channel processor then provides a processed output signal to a video combiner. The combiner combines several processor output signals as well as their modified key signals and provides a combined output signal--all in response to, and under the control of, priority signals received from an operator interface module.
Special video effects like wipes or dissolves are obtainable by adjusting values of a video input source signal. A two input switcher including a mixer can adjust the video input values (a) by multiplying, which is also called cutting in the art, a first input video signal by a mix coefficient .alpha.; and (b) by multiplying a second input video signal by the complement of the mix coefficient, i.e. by (1-.alpha.); and (c) by combining by way of summing the two products. Typically, wipes and dissolves differ in the time interval over which the mix coefficient .alpha. changes. For example, on the one hand, if the mix coefficient .alpha. changes over a time interval that is much shorter than a video field interval, there typically being two fields in a frame, then the mixture is a wipe. A wipe can give a visual perception similar to that of a vertical line moving across a television screen in such a manner that an existing picture is replaced, for example, in a generally left-to-right direction by a new picture. On the other hand, if the mix coefficient .alpha. changes over a time interval that is equal to or longer than a video field interval then the mixture is a dissolve, which is also called a "fade" in the art. A dissolve can give a visual perception similar to that of one image replacing another image on the screen. Stated in an alternative way, in a wipe, the mix coefficient tends to differ among the different pixels that comprise the picture whereas, in a dissolve, the mix coefficient tends to be the same for all pixels that comprise the picture.
Other special video effects can be obtained by changing a priority signal. For example, in response to differing values of a priority signal, a composite video picture can be changed so that a component scene which is the foreground scene of a picture at one point in time becomes the background scene of the picture at another point in time. U.S. Pat. No. 4,758,892, which issued Jul. 19, 1988 and which is entitled "System for Producing a Video Combine from Multiple Video Images" describes a switcher including a combiner that combines two or more channels of video signals in such a way as to make scenes or images corresponding to the video signals appear to be one behind the other as in accordance with a selected priority. That combiner may result in a picture which displays a first video image from one channel of highest priority in the foreground, followed by a second video image from a second channel, which in turn is in front of, and followed by, a third video image from a third channel, etc.
U.S. Pat. No. 5,231,499, which issued Jul. 27, 1993 to the present applicant and which is entitled "Keyed, True-Transparency Image Information combine" and which is incorporated herein by reference, discloses another combiner structure that dynamically rank orders the individual input image signals; generates true transparency image key signals; and processes background key signals. Keyer units are switchably changed to produce true transparency processed key signals such that a keyer unit can modify the value of its processed key signal with the values of the input key signals in higher priority channels to generate a true transparency processed key signal for a j-th priority channel. The combiner also cuts each respective input image signal V.sub.i and background signal V.sub.B by its respective processed key signal and combines the plurality of cut signals to generate an output image combine signal V.sub.C which is usable in displaying a special video effect.
A border, or boundary, area of a picture scene can be defined by reference to the transition between one region of the picture where a key, or mix coefficient .alpha., has a value of one and another region where the key .alpha. has a value of zero. Borders, or edges, or boundaries, or transition regions, are not always that distinct. At times a transition may occur in passing from one region of a picture across a boundary to another region of the picture. For example, a border may be visually transparent, or "soft," which means that underlying parts of the picture will actually remain visible through a superimposed picture.
Unfortunately, known switchers typically produce an artifact when opaque priority elements are wiped on or off with a soft border and in particular when opaque images are visually located behind the border. Within the soft border region, an opaque element commonly becomes transparent, thereby revealing any lower priority elements, or scenes, behind it. This is optically incorrect and therefore is considered to be a problem which needs a solution. The problem arises because known switchers modify the input key signals by changing the transparency characteristic of the video signal. What results is a transparent element which was and should be an opaque element.